Edge lighting techniques for illuminating dials, indicators and instrument face plates have become increasingly popular over the last decade because they offer opportunities to mask the light source from the viewer or operator and also flexibility in positioning the light source, and if properly utilized, can defuse and spread light evenly over the surface to be illuminated.
A variety of techniques have been used in edge lighting systems to achieve these ends. Edge lighting is basically the utilization of a transparent plastic conductor that acts as an optical receptor for receiving light from a source, and an optical emitter for directing light within the conductor in some coherent manner across the surface or area to be illuminated.
One technique for increasing the receptability of the conductor to light is to provide an enlarged isosceles triangular section on the conductor adjacent the light source. Another is to mount the light source in a circular aperture in the conductor and to apply white paint to the periphery of the conductor. Still another technique is to provide a forty-five degree bevel along one or more edges of the conductor, with the bevel painted white, and to mount a backplate with a background color directly behind the conductor, with a bezel covering the bevel and the light source positioned directly behind the beveled surface. This bevel acts to direct light ninety degrees from the source through the transparent conductor illuminating the rearwardly mounted background color.
Where the space for a lamp is limited beyond the periphery of the piece, this forty-five degree facet cut along the edge permits lamp location behind the edge rather than beyond it. However, bezels and masks must be used to block unwanted stray light. Sometimes white paint or metallizing on the forty-five degree facet improves performance by reflecting rays striking the bevel at less than the critical angle for total reflection.
The local destruction of polished surfaces on the conductor, such as by frosting, sandblasting, scratching or honing will cause light rays to scatter as they strike that zone. A few rays will pass through the frosted zone and produce a glow but most will scatter diffusely back across the thickness of the conductor and many will emerge or emit from the opposite surface to be viewed as an illuminated marking.
Where dark backgrounds are desired, a second opaque part is sometimes used behind the rear surface of the conductor. It may be almost in contact with the conductor, but a thin film of air desirably remains between it and the edge lighted panel to preserve the air interface for internal reflectivity.
As an alternative to utilizing a beveled surface to deflect light from a rearwardly positioned light source, it is also known to use a constant thickness curved conductor for the same purpose.
One specific conductor for edge lighting instruments is an annular illumination ring that extends about the periphery of a dial face to be illuminated. It has a light receiving flat radial surface adjacent the light source and an outer forty-five degree beveled surface that extends axially from one end of the ring the other. The light receiving portion also serves to center the dial face to be illuminated because the ring is not radially expandable. Since the light receiving portion of the ring is a single flat radial surface, its light receiving capabilities are relatively low. And the light transmitting capability of the ring is limited because the bevel directs light laterally in the light receiving portion of the ring where it is ineffective to illuminate the face of the dial.
Because of the limited light conducting capabilities of this ring, it is necessary in this illumination system to make the dial face itself translucent so that light passes directly through the dial to assist in illuminating the dial face. The problem is that the opaque areas on the dial face cause the dial face to be illuminated unevenly with many dark spots. Such an illuminated instrument is manufactured by U. S. Gauge Company.
It is a primary object of the present invention to ameliorate the problems discussed above in illuminating instrument indicating panels by edge lighting.